1. Field of the Invention
The present invention relates to a spindle motor, particularly, relates to a spindle motor having a dynamic pressure fluid bearing.
2. Description of the Related Art
Since a spindle motor having a dynamic pressure fluid bearing is excellent in characteristics while rotating at higher speed, such a spindle motor is mostly utilized for driving recording mediums such as magnetic discs and optical discs in a recording and reproducing apparatus.
One example of conventional spindle motors having a dynamic pressure fluid bearing was disclosed in the Japanese publication of unexamined patent applications No. 2004-11897. The spindle motor disclosed in the Japanese publication of unexamined patent applications No. 2004-11897 is provided with a pair of radial bearing sections, which is constituted by a shaft and a sleeve, and a thrust bearing section, which is constituted by an upper surface of the sleeve and an internal surface of a hub.
On the other hand, besides the spindle motor in the above-mentioned configuration, another spindle motor that is provided with two thrust bearing sections has been studied. The other spindle motor is constituted such that a flange protruding outside a sleeve and a seal plate are disposed in an axial direction in the sleeve side with being apart from each other so as to be excellent in vibration tolerance and shock resistance, which are required for portable equipment.
Further, the spindle motor is constituted such that a thrust ring provided in a hub side is sandwiched between the flange and the seal plate. Consequently, the two thrust bearing sections are constituted by upper and lower two surfaces of the thrust ring and surfaces of the flange and the seal plate, which confront with the upper and lower two surfaces of the thrust ring respectively.
With referring to FIG. 10, a spindle motor used in a hard disc drive (HDD) for one-inch disc is depicted as one example of such a spindle motor mentioned above.
FIG. 10 is a cross sectional view of a conventional spindle motor according to the prior art. In FIG. 10, a spindle motor 151 is composed of a stator 114 and a rotor 112.
The stator 114 is further composed of a motor base 108, a sleeve 104, which is fixed to the motor base 108 and formed with a flange section 104a that is disposed in one end portion of the sleeve 104, and a core 109.
Further, a coil 110 is wound around the core 109, and a seal plate 117 in a ring shape is fixed to the other end potion of the sleeve 104.
On the other hand, the rotor 112 is further composed of a hub 107, wherein a ring magnet 111 is fixed on an outer circumferential surface of the hub 107. The hub 107 is integrally formed with a shaft 101 in the middle of the hub 107. An outer cylindrical section 113 is engaged with and fixed on an outer circumferential surface of the shaft 101.
Further, a thrust ring 103 is fixed on an inner circumferential surface of the hub 107.
Furthermore, the outer cylindrical section 113 of the rotor 112 is inserted into a center hole of the sleeve 104 of the stator 114.
Accordingly, the rotor 112 is sustained by the stator 114, and results in being rotatable freely with respect to the stator 114 through a dynamic pressure fluid bearing to be detailed next.
Thrust dynamic pressure fluid bearings SB11 and SB12 are constituted by the thrust ring 103, the flange section 104a, the seal plate 117 and lubrication fluid (hereinafter referred to as lubricant) filled in gaps among them.
More specifically, a pair of dynamic pressure grooves (not shown), which are formed on both of top and bottom surfaces of the thrust ring 103 in the axial direction, generates a dynamic pressure in accordance with revolution of the rotor 112 and exhibits a function of thrust bearing.
In other words, a dynamic pressure, which raises the rotor 112, is generated by a bottom surface of the thrust ring 103 and a top surface of the seal plate 117 and another dynamic pressure, which lowers the rotor 112, is generated by a top surface of the thrust ring 103 and a bottom surface of the flange 104a, and balancing both the dynamic pressures makes the rotor 112 float and hold with respect to the stator 114.
Radial dynamic pressure fluid bearings RB11 and RB12 are constituted by the outer cylindrical section 113, the sleeve 104 and lubricant filled in gaps between them.
More specifically, dynamic pressure grooves, which are formed on either surface of an outer circumferential surface of the outer cylindrical section 113 and an inner circumferential surface of the sleeve 104, generates a dynamic pressure in accordance with revolution of the rotor 112 and exhibits a function of radial bearing, wherein the outer circumferential surface of the outer cylindrical section 113 confronts with the inner circumferential surface of the sleeve 104.
The radial dynamic pressure fluid bearings RB11 and RB12 are provided in pairs with being apart from each other in the axial direction.
In the above-mentioned configuration, a filling route of lubricant is formed so as to link the two thrust dynamic pressure fluid bearings SB11 and SB12 and the two radial dynamic pressure fluid bearings RB11 and RB12 with connecting them in series.
Further, the dynamic pressure generated by the thrust dynamic pressure fluid bearings SB11 and SB12 is directed toward a direction of so-called “pump-in” so as to prevent lubricant from leaking out from the thrust dynamic pressure fluid bearings SB11 and SB12 while the rotor 112 rotates.
In other words, the dynamic pressure generated by the thrust dynamic pressure fluid bearings SB11 and SB12 is generated so as to direct the lubricant inward to the radial dynamic pressure fluid bearings RB11 and RB12 if it is described along the filling route of the lubricant.
In the above-mentioned spindle motor having two thrust dynamic pressure fluid bearings, a groove shape of dynamic pressure grooves provided in the two thrust dynamic pressure fluid bearings respectively is formed with having slight difference between them due to variations in fabricated dimensions and variety of materials when manufacturing. If the difference exceeds a certain level, a generated dynamic pressure creates further difference that is not to be ignored. Particularly, the higher the rotational speed of the rotor is, the more the difference is made remarkable.
More specifically, in some cases, a rotor may be excessively raised or lowered by the unbalanced dynamic pressure, and resulting in a problem such that the rotor may accidentally contact with a stator.
Further, a thrust dynamic pressure fluid bearing disposed outside the filling route of lubricant, that is, the thrust dynamic pressure fluid bearing SB12 constituted by the bottom surface of the thrust ring 103 and the top surface of the seal plate 117 is formed so as to “pump-in” the lubricant. At the same time, another thrust dynamic pressure fluid bearing disposed inside the filling route of lubricant, that is, the thrust dynamic pressure fluid bearing SB11 constituted by the top surface of the thrust ring 103 and the bottom surface of the flange 104a is also formed as the “pump-in” configuration so as not to “pump-out” the lubricant although a groove shape of dynamic pressure grooves provided in the two thrust dynamic pressure fluid bearings respectively is formed with having slight difference between them due to variations in fabricated dimensions and variety of materials when manufacturing.
On the contrary, in case revolution of the rotor is in higher speed, particularly, a “pump-in” pressure caused by the two thrust dynamic pressure fluid bearings increases.
Further, centrifugal force is added to the inside thrust dynamic pressure fluid bearing, and resulting in increasing a pressure of lubricant filled in the dynamic pressure fluid bearings excessively more than necessary.
As a result, force that separates the rotor 112 from the stator 114 increases excessively and the force defeats another force, which is generated by the inner thrust dynamic pressure fluid bearing and functions so as to make the rotor 112 lower, and resulting in a problem such that the rotor 112 is made contact with the stator 114.
On the other hand, lubricant has a particular temperature characteristic: the lower the liquid temperature is, the higher the viscosity is. Consequently, a “pump-in” pressure increases in accordance with lowering temperature. In some cases, a rotor is raised excessively by increasing “pump-in” pressure, and resulting in contacting the rotor with a stator.
On the contrary, viscosity decreases in accordance with rising liquid temperature, so that the “pump-in” pressure decreases. In some cases, the rotor is lowered excessively, and resulting in contacting the rotor with the stator.
In other words, the rotor possibly contacts with the stator in case liquid temperature, that is, ambient temperature changes heavily.
If the rotor contacts with the stator, load is added to revolution of the rotor, and resulting in increasing power consumption of the spindle motor.
Further, contacting the rotor with the stator creates another problem related to reliability such that vibration of a shaft increases and resulting in shortening a life of the spindle motor.